Full-color light emitting diode (LED) displays in which LEDs emitting light at different wavelengths are grouped into pixels have been proposed as potential replacements for displays using LEDs as backlight light sources. Each pixel consists of red, green, and blue LEDs or red, green, blue, and white LEDs. In such an LED display, red, green, and blue LEDs are fabricated in packages and are mounted on a substrate. However, due to the large distances between the constituent LEDs of each pixel, high-quality resolution is difficult to obtain. Pixels consisting of packages of LEDs are difficult to apply to micro-LED displays that have recently received much attention. LED pixel units have also been proposed in which red LEDs, green LEDs, and blue LEDs constituting one pixel are mounted in one package. In such an LED pixel unit, the distance between the adjacent LEDs (i.e. sub-pixels) in one pixel is small but the distance between the adjacent pixels is difficult to reduce. Further, light interference may occur between the red, green, and blue LEDs.
Thus, for the purpose of reducing the distance between pixels, an attempt has been made to fabricate an LED display module in which groups of LED chips, each of which includes red LED, green LED, and blue LED chips, are arrayed in a matrix on a substrate. It is, however, difficult to mount the LED chips at predetermined heights and intervals on the micrometer-sized substrate. Different heights and intervals between the LED chips mounted on the substrate deteriorate the color reproducibility of the LED display module. Wire bonding is necessary for electrical connection between electrode pads and the LED chips on the substrate but it takes at least tens to hundreds of hours to manufacture one product.
Particularly, in the course of mounting tens to hundreds of LED chips on the substrate, there is a high possibility that some of the LED chips may not be accurately located at desired heights from the substrate at desired positions on the substrate. In this case, a designed light emitting pattern cannot be achieved, resulting in poor color reproducibility. Further, the LED chips are required to emit light at various wavelengths and should be divided on the basis of wavelength. For example, the LED chips emitting light at different wavelengths should be divided into unit groups. Thereafter, the unit groups should be arranged, making it troublesome and difficult to fabricate the LED display modules.